Data processing systems are composed of a plurality of functional units, generally in the form of integrated circuits distributed on electronic cards or printed circuit boards. These units (cards and/or integrated circuits) typically operate in synchronism under the control of clock signals supplied from a common-base clock.
By comparison with asynchronous systems, synchronous systems have an advantage in speed, as exchanges of information signals between the units are performed directly, bypassing resynchronization circuits which may introduce delays. Another advantage is the ability to freeze the system in a given logic state by stopping transmission of clock signals. Synchronous systems also simplify certain maintenance operations, such as reducing the frequency when defective electronic cards are replaced, as described in French Patent Application No. 88 16194 filed on Dec. 9, 1988, (U.S. patent application Ser. No. 07/444,941, filed Dec. 4, 1989); entitled "Systeme electronique a plusieurs unites amovibles" ("Electronic System with Several Detachable Units").
On the other hand, synchronous systems have the disadvantage of being difficult to implement, particularly when there is a large number of synchronous units. It is difficult to achieve fully synchronous operation of all the units while maintaining a high operating frequency. Links between units and technological variations in these units introduce spurious synchronization differences which make it necessary to reduce the operating frequency. These spurious phase shifts are both static ("skew") and dynamic ("jitter").
To overcome this drawback, the first approach is to eliminate the causes of phase shift due to the links by disposing the units as regularly as possible in the system. However, this solution does not make up for technological variations which may be very large, particularly if the integrated circuits come from different manufacturers.
Hence, it has been necessary to provide correction circuits that act on the characteristics of the transmitted signals. Of course, to resolve the synchronization problem, the clock signals must be corrected as the first priority. Generally, clock signals are generated by clock generators from a basic clock signal furnished for example by a quartz oscillator. The correction can be made to these generators by influencing the phases of the transmitted signals and in particular the amplitude (voltage or current) of these signals by adjusting the characteristics of the generator output amplifiers.
To determine the optimum corrections, the safest solution is to run each system on a test bench and search for the correction that produces an amplitude and phase match between the signals actually obtained and reference signals. These reference signals define characteristics imposed by the specifications of the system being designed. Also, in order to guard against problems linked to technological variations, it is preferable to choose a digital adjustment method whereby the adjustments to be made are defined by a binary number defining the adjustment value. To make the correction according to the adjustment value, discrete delay circuits and adjustable amplifiers are used.
As soon as the values of the adjustment parameters are determined on the test bench, they must be preserved for use in normal operation after the system has been initialized. Generally, systems of some size include maintenance devices composed essentially of a service processor, specific maintenance units, and maintenance circuits integrated into functional units. These elements are connected together by a set of links called a maintenance channel. The system is then started up under the control of the service processor which triggers initialization of the technological parameters controlling the various adjustment means provided in the clock generators. For this purpose, a nonvolatile memory belonging to the service processor (diskette or other auxiliary memory) in which the adjustment parameters are stored can be used. However, this solution has the drawback that replacement of certain system circuits may make it necessary to change the stored data. This is particularly true in the case of a changed clock generator. Thus, the goal of the invention is to propose a solution to overcome the above drawbacks.